This invention relates to a device for detecting the length of cracks of a fatigue test piece.
As a method of finding the bahavior of fracture of a component part, there has heretofore been developed a "K" value control device for displaying the speed of development of cracks of the component part due to its fatigue in terms of a stress intensity factor ("K" value) and controlling the load imposed thereon based on the factor or "K" value.
This control device is a fatigue test apparatus arranged to follow after the cracked leading end of a fatigued test piece such as flat metal sheet etc. by means of a chasing detector means, compute "K" value corresponding to the length of the crack and apply the resulting value of load on the test piece.
A hypothesis has been advocated in which the development of cracks in a component part is caused by a local stress exerted in the leading end of the crack and is determined thereby in principle. Stating in concrete manner, a stress intensity factor ("K" value) is given which is determined by the shape of the test piece, the load applied thereon and the length of the crack.
According to the hypothesis, if the amplitude of .DELTA.K given by a fluctuation of the load is kept constant, then there is obtained a relationship between the stress intensity factor ("K" value) and the length of the crack "a" in which the speed of development or propagation of the crack da/dN is kept constant, where "N" is the number of repetitions of application of the fluctuating load.
The following formula is given for example to calculate the tensile load. EQU P(.lambda.)=twk/.sqroot.a(1+0.5948.lambda..sup.2 +0.4812.lambda..sup.4 +0.3963.lambda..sup.6 +0.3238.lambda..sup.8)
where
.lambda.=2a/w, PA1 t=thickness of plate, PA1 w=width of plate, PA1 2a=length of crack, PA1 k=stress intensity factor, PA1 P(.lambda.)=tensile load.
Further, the "K" value control device is required to have such capacities as being capable of detecting the end of the crack and taking or reading out the location thereof as a signal, computing the load corresponding to a spontaneous value of .DELTA.K based on the length of the crack and the shape of the test piece and applying the indicated load on the test piece.